Theme setting not remembered

### Describe the bug

After the change to the new command `/settings theme `, I have trouble getting it to stay on my default. I keep having to select the default theme instead of the blue github theme.

### Affected version

GitHub Copilot CLI 1.0.69-0

### Steps to reproduce the behavior

1. Open copilot on new codespace
2. Theme setting is "github" not "default"

### Expected behavior

Default theme is default

### Additional context

_No response_

Start by locating the `/settings theme ` command handling and reproduce the behavior by opening Copilot in a new codespace. Check the setting's persistence and verify that a new session retains the selected default theme instead of changing to the github theme.

索引モデルが issue の本文から書いたものです。